There is a variety of applications that require the use of a uniform line image formed from a coherent light beam. One such application is laser thermal processing (LTP), also referred to in the art as laser spike annealing (LSA) or just “laser annealing,” which is used in semiconductor manufacturing to activate dopants in select regions of a semiconductor wafer when forming active microcircuit devices such as transistors. Laser annealing uses a scanned line image from a light beam to heat the surface of the wafer to a temperature (the “annealing temperature”) for a time long enough to activate the dopants but short enough to minimizing dopant diffusion. The time that the wafer surface is at the annealing temperature is determined by the power density of the line image, as well as by the line-image width divided by the velocity at which the line image is scanned (the “scan velocity”).
To achieve high throughput in a commercial LSA tool, the line image needs to be as long and as narrow as possible, while also having a high power density. An example range for usable line-image dimensions is 5 mm to 100 mm in length, by 25 microns to 500 microns in width. To achieve uniform annealing, it is also necessary for the intensity profile along the length of the line image to be as constant as possible. Typical semiconductor processing requirements call for the anneal temperature to be between 1000° C. and 1300° C. with a temperature uniformity of +/−3° C. To achieve this degree of temperature uniformity, the line image formed by the annealing light beam needs to have a relatively uniform intensity, which under most conditions is less than +/−5%.
A CO2 laser is a preferred light source for laser annealing applications because its wavelength (nominally 10.6 microns) is much longer than the size of devices irradiated, resulting in less scatter and more uniform exposure, thereby eliminating “pattern effects,” and because it emits a relatively high-intensity beam. However, the coherence length for a CO2 laser is relatively long, typically several meters, making it unfeasible to use a binary optic approach to produce a long, narrow line focus with ˜10% (+/−5%) uniformity based on principles of Kohler illumination.
Traditional methods of performing laser annealing with a CO2 laser including imaging the light beam onto a pair of knife-edges. The knife-edges are positioned to transmit only a narrow central portion of a Gaussian light beam. The length of the transmitted light beam is that portion of the beam that meets the uniformity specification for the line image formed at the wafer. For the Gaussian CO2 light beam, the knife edges are typically arranged to pass only the middle 10% of the Gaussian beam. Unfortunately, this means that the other 90% of the light beam is rejected, which is a very inefficient use of the high-intensity light from the light source.